The stability of omeprazole in the aqueous solutions containing arginine or sodium phosphate dibasic(SPD) was examined at 30, 40 and $50^{circ}C$. Arginine or anhydrous SPD was added to omeprazoie solution ($200{mu}g/;ml$ in distilled water) to yield $100{mu}g/;ml$ concentration of each. Then, the solution was kept at 30, 40 or $50^{circ}C$ for 90 hrs. Aliquots of the solution were withdrawn at specified time intervals and assayed by HPLC for intact omeprazole. The remaining percentage-time curves revealed that omeprazole was degraded rapidly as funtions of time and temperature following pseudo first-order kinetics. The rate constant in the SPD solution was much higher than in the arginine solution. In other words. the degradation half-lives of omeprazole at $30^{circ}C$, for example, was 148 and 76 hr in arginine and SPD solutions respectively. The initial pH of the solution containing $100{mu}g/;ml$ of arginine or SPD was 9.7 or 8.7, respectively. Since omeprazole is more stable as the pH of its solution becomes more alkaline, the longer half-life of omeprazole in arginine solution could be explained by the more alkaline characteristics of arginine than SPD in the solution. The activation energy necessary for the degradation reaction was almost identical in both solutions, indicating similar degradation mechanisms of omeprazole in the solutions. In conclusion, omprazole was more stable in the presence of arginine than of SPD.